Oil and gas production processes generate large volumes of liquid waste. Hydraulic fracturing of shale utilizes high-pressure water to fracture shale formations. The wastewater generated during the drilling phase is called flowback water, whereas the water generated during the production phase is called produced water. Both the flowback and produced waters contain various organic and inorganic components, and discharging produced water can pollute surface and underground water and soil. Since approximately 250 million barrels per day (i.e., ˜30 million m3 per day) of produced water are generated globally, an amount that is expected to continue increasing for an extended period of time, there is a growing need for new methods to treat large volumes of produced water robustly and efficiently.
A variety of methods are currently utilized to treat produced water for the purposes of discharge as well as for recycling and reuse in subsequent hydraulic fracturing operations. This diverse set of water treatment techniques include de-oiling (removing dispersed oil and grease), removal of soluble organics, disinfection, suspended solid particle removal, dissolved gas removal (including hydrocarbon gases, carbon dioxide, and hydrogen sulfide), desalination (removing sodium and chloride ions), and water-softening (reducing calcium and magnesium hardness), among others. High voltage (HV) plasma discharges have been studied in application to water for various parameters including removal of dispersed oil/grease and soluble hydrocarbons, water softening, and disinfection.
Plasma is ionized gas. One of the simplest ways to produce a plasma discharge in air is to utilize two electrodes (i.e., a cathode and anode) closely placed in air, i.e., 2-5 mm. When the voltage between the two electrodes increases to a certain value such as 2 kV, breakdown of air between the two electrodes takes place, generating a discharge of plasma. Depending on the magnitude of the voltage across the two electrodes and electrode geometry, a number of different types of plasma discharges can be produced, including corona, spark, and arc type plasmas, among others.
When discharging plasma in liquid, the process can be more complicated if the electric conductivity of the liquid is significantly greater than that of gas. For example, when one tries to generate plasma discharges in municipal tap water, the electric conductivity of tap water (i.e., approximately 0.2-0.8 mS/cm) causes the water to behave as an electric conductor, creating a path for electrons (i.e., a short circuit) thereby preventing breakdown and discharge of plasma in water even if the distance between the two electrodes is kept relatively small such as 3 mm.
In this background example, after applying voltage to municipal tap water for an initial period of time of approximately 10-20 s, a large number of gas bubbles are generated from both electrodes, due to electrolysis. When the gas generated by electrolysis of water between the two electrodes reaches sufficient volume, breakdown can occur, and subsequently plasma is discharged in water. Once the initial breakdown and plasma discharge takes place, the high temperature of plasma (approximately 2,000 K) produces additional gas bubbles between the two electrodes, and plasma discharge can be sustained.
When the electric conductivity of liquid is significantly larger than that of typical municipal tap water, plasma discharge typically does not take place unless gas is supplied to the water at the electrode from an external source such as a compressor or gas tank. In the case of industrial wastewater produced from hydraulic fracturing of shale for oil and gas (produced water), the electric conductivity is very high, approximately 200 mS/cm due to a large amount of dissolved ions such as sodium, calcium, chloride, magnesium, etc. (see Ahmadun et al., “Review of technologies for oil and gas produced water treatment,” Journal of Hazardous Materials, vol. 170, pp. 530-551, 2009). Due to this high electrolytic conductivity, produced water provides a special challenge for the generation of plasma discharge, but at the same time unique opportunity, if one can successfully discharge plasma in produced water, that can be applied for the removal of dispersed oil/grease and soluble hydrocarbons (see McIntyre et al., “Uses of ultraviolet/ozone for hydrocarbon removal: Applications to surfaces of complex composition or geometry,” Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, vol. 9, pp. 1355-1359, 1991), water softening (see Yang et al., “Removal of CaCO3 scales on a filter membrane using plasma discharge in water,” Int. J. Heat Mass Transfer, vol. 52, pp. 4901-4906, 2009; and Yang et al., “Mineral Fouling Control by Underwater Plasma Discharge in a Heat Exchanger,” ASME Journal of Heat Transfer, vol. 133, p. 054502, 2011), and disinfection (see Kim et al., “Concentration of hydrogen peroxide generated by gliding arc discharge and inactivation of E. coli in water,” International Communications in Heat and Mass Transfer, vol. 42, pp. 5-10, 2013).
In such high electric-conductivity liquid, electrons instantly and continually flow from cathode to anode as the high electric conductivity liquid provides an effective path for electrons to flow, a phenomenon that can be referred as leakage between the two electrodes. In such cases, the injection of gas between the two electrodes can create a barrier to stop the flow of electrons between the two electrodes, assisting in the process of breakdown of liquid, such that plasma discharges can be generated in high electric-conductivity liquid using moderately high voltages of 1-3 kV.
When gas injection is utilized to assist the generation of plasma discharges in high electric-conductivity liquid, it is essential to have sufficiently large-size gas bubbles that can fill the space between the two electrodes. Typically, assuming the gap distance used between the two electrodes in liquid is 2-3 mm, it is desirable to generate gas bubbles of at least 2-3 mm or greater to ensure that plasma discharges take place between the electrodes in liquid.
Flow rates in the treatment of produced water can be in a wide range from 1 L/s to 300 L/s. For efficient treatment of larger produced water flow rates, there is a concomitant need to have increasingly large plasma discharges. If the plasma discharge can be stretched from within a 2-mm space to a much larger liquid space, for example, >5 cm in length, treatment efficiency can be increased dramatically.
The high electric conductivity of produced water that makes plasma difficult to ignite, as produced water often behaves as a short circuit. However, liquids having extremely low electric conductivity, as in the case of hydrocarbon fuels such as jet fuel (JP-8) and diesel, also present challenges in discharging plasma. What is needed in the art are improved systems and methods for stretching plasmas in both high and low conductivity liquids.